I'm gimpy enough that I don't get out by myself much since DS1 got mobile/too heavy to sling, but I've seen a bit of NIP here in Cleveland, and have been seen by others often also to the point that I get asked for breastfeeding advice by pregnant and new moms that I've just met for the first time, at least once every couple months that happens.
Especially when slinging a little one, it's hard to tell sometimes that a mom is nursing - I've had it happen several times that I was nursing the baby in the sling while using a motorized scooter around a store and people have looked down at the baby - whose head is not covered and face is visible - and not realize that he was nursing and snoozing. I keep my shirt pulled down to his cheek for my physical comfort (baby nails are sharp!) not for modesty and apparently it's way more discrete that way than trying to hide it LOL
I do hear from other moms how they "never see anyone NIP" and I haven't seen the advice I give them so here it is - start taking note not of whether the baby is actively nursing, but of what luggage the mama's carrying. If baby's in a sling and mama's carrying a purse that is either too small to hold more than her wallet, phone, and a disposable diaper or alternatively a huge artisan-made-looking beechcomber bag that is clearly one large pocket with no way to hold a bottle upright, in my experience you've got about a 95% likelihood that you've just spotted a breastfeeder. If baby is in one of those mega strollers, no bottle in hand, small bag (possibly a diaper bag) with a adult water bottle in the side - about 85% chance it's a breastfeeder. Older baby (as in able to sit up unaided) and mama carrying nothing more than a very small purse? Breastfeeder and possibly ECer (or just uses really small sposies, depending on purse size - those sposies really can be rolled up pretty small tho). And don't assume that the carrier carseat ones are all bottle-fed either - of the families I know who use the carrier style carseats (mine included) about 90% breastfeed (tho the subject pool is rather crunchy)
The only way you can be absolutely certain that you've got a bottlefeeder in front of you is if they have an actual bottle present, but then you can't be sure it's not EBM or, with an older baby, a supplimental juice/water bottle for hydration. Certainty of ABM is only achieved when you see either an obvious single-serve premixed bottle there, or see her shaking the powder into water.
If you start looking at the luggage, you might wind up rather surprised at the number of mom/baby dyads you'll suddenly realize most likely are breastfeeders.
As for how often I see it in public, that increased when I knew what signs to look for. I've seen it at local big box stores and fabric stores fairly often, considering I don't see babies bottlefed in those much either. I've met other sling-wearing mamas at my local JoAnn fabrics several times. And my UU church is predominantly breastfeeders, it's more of a surprise to see a baby get a bottle there than it is to see one breastfed. I'm frequently one of 3 or 4 nursing mamas within the "family section" of the sanctuary (we self-segregate to be able to make a quick step out if our little ones get really vocal/fidgety, tho we have a hard time keeping the little old folks OUT of the family area even tho the church administration has marked the pews to ask that they be reserved for families with small children... too many of the grammies and grampies ENJOY being around the little ones).